Iranian President Hassan Rouhani will undoubtedly play the well-dressed matinee idol in this year’s UN annual gabfest, which begins Tuesday. But will Israel’s Benjamin Netanyahu be the only one to note that this emperor has no clothes?

Turtle Bay diplomats are eager to hear His Moderation’s every siren song. In Washington, too, aides to President Obama are hailing “positive developments” in Tehran since Rouhani took office last month.

Speaking with reporters Friday, National Security Adviser Ben Rhodes didn’t exclude the possibility of an unscheduled Obama-Rouhani “bilateral” this week in New York. Even a casual corridor handshake between Secretary of State John Kerry and his counterpart, Javad Zarif, would be a first. Relations have been that cold ever since the 1979 Iranian revolution.

Yes, it’s springtime for diplomacy. Obama’s “no style points” foreign policy, we’re told, is making strides — or is it his “credible threat of force” in Syria, as Rhodes said Friday?

Rouhani, a skilled negotiator, represents an Iranian faction, led by Akbar Hashemi Rafsanjani, which fears the mullah regime’s collapse — unless it gets an end to the various UN, US and European sanctions imposed over its nuclear program.

Hence the unprecedented Iranian charm offensive. Rouhani not only released some political prisoners, he penned a Washington Post oped (markedly nicer than Vladimir Putin’s New York Times column) in which he offered to mediate the Syrian crisis. He even tweeted Rosh Hashanah greetings to world Jewry. Plus, his delegation for the UN festivities will include Siamak Moreh Sedgh, the Iranian parliament’s token Jew.

See? No anti-Semites in Tehran.

(And never mind that the presence of a Jew in the Majlis hasn’t stopped Iran from hanging Jews in Tehran’s town square on phony espionage charges. Or that, when NBC asked him about the Holocaust last week, Rouhani’s answer was that he’s no historian — that is, a dodge of the whole Holocaust-denial question.)

Will Obama fall for it?

The doves swooning over Rouhani seem to think Obama is on a roll, after his Syria zigzagging wound up with a supposedly stellar diplomatic pact.

Really? Kerry and his Russian counterpart, Sergey Lavrov, signed their framework agreement on Sept. 14. But deadlines are alredy being blown: The Security Council resolution that was supposed to make the deal binding was scheduled to be voted on this past weekend, but it’s still pending.

When and if it finally gels (in days or even weeks), it’ll likely fail to include any serious enforcement mechanisms.

To recap: Kerry negotiated a deal with Lavrov. Now the US is negotiating (again) with Russia and others at the UN. Then we’ll negotiate with Syria — or, actually, Hague-based UN inspectors will negotiate with Syria. By then, well, who’ll remember the whole affair.

Meanwhile, not one canister of Syria’s Sarin has been destroyed. It’ll be a while, if ever. Meanwhile, Bashar al-Assad is actually safer since the August atrocity.

Will the Iran talks play out as badly?

Mind you, we’re not even negotiating with Iran’s real decider, Supreme Leader Ali Khamenei. He’s letting Rouhani play diplomatic games for now, but his hardliners see acquiring nukes central to the regime’s survival. It’s far from clear that he’ll sign off on any deal that Rouhani reaches with the West.

Then again, what would that deal look like?

In the past, Obama indicated he’d be open to taking all of Iran’s uranium that had been enriched to near weapon grade (20 percent) out of the country. But according to former International Atomic Energy Agency inspector Olli Heinonen, Tehran’s program is too advanced now for that to do much good.

Iran now has so many advanced centrifuges (and so much partly-enriched uranium) that it could quickly replace any fuel it hands over. Plus, it has also launched an entirely different, plutonium-based, system.

Israel’s minister of strategic affairs, Yuval Steinitz, warned Friday that Iran is on course to develop a nuclear bomb within six months. “There’s no more time to hold negotiations,” he told the daily Israel Hayom.

But Obama adviser Rhodes says there is time. The White House claims it will judge Iran by deeds, not words — but nonetheless seems enchanted by Tehran’s sweet talk.

So will Netanyahu be the skunk in this week’s garden party? Bibi’s aides had told me he’d keep a somewhat low profile this year. But alarm bells over US-Iran talks are ringing loudly all over Jerusalem, so maybe not.

Of course, with the appetite for appeasement so strong at the moment, it’ll be tough for Bibi to get any other leaders to stop admiring the emperor’s new clothes.